Indenofluorenes for organic optoelectronics: the dance of fused five- and six-membered rings enabling structural versatility

Polycyclic pi-conjugated hydrocarbons (PCHs), either unfunctionalized or structurally modified derivatives, have attracted tremendous interest in the past few decades as high-performance semiconductors for use in new generations of organic (opto)electronic devices. Among several PCHs realized to date, the 6-5-6-5-6 pi-fused-ring backbone of indenofluorene (IF) stands out as a unique semiconducting architecture with great structural and property versatility affording six different regioisomers, diverse functionalization/substitution positions, pi-conjugation/delocalization patterns, aromatic behaviors, and electronic structures. In this review, we summarize and analyze the historical and recent advances in the design and implementation of IF-based semiconductors in organic transistor and solar cell devices, as well as in understanding the chemical structure-molecular property-semiconductivity relationships. Following an introduction to the fascinating properties of an IF pi-framework that distinguishes this core among PCHs, we present IF-based semiconductors and discuss their properties by classifying them into four main families (IF-diones, IF-DCVs/IF-TTFs, pi-IFs, and (un)substituted DH-IFs) considering whether methylene or methine C-bridges are present and how these positions are functionalized or substituted. For each family, design and synthetic approaches, molecular properties, and transistor/solar cell device applicability and/or performance are reviewed and discussed. At the end, we conclude with a section discussing the challenges and opportunities for future progress of IF-based semiconductor materials and related (opto)electronic technologies.